Sexual reproduction of scleractinian corals in northern Taiwan and roles of Dmrt gene family in coral reproduction

• Main Authors: Chieh-Jhen Chen, 陳倢蓁
• Other Authors: Wei-Jen Chen
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/j526xq

博士 === 國立臺灣大學 === 海洋研究所 === 105 === Coral reef ecosystem is now facing severe threats due to irreversible disturbances caused by natural disasters, anthropogenic effects and global warming. Coral communities in Taiwan are found in both subtropical and tropical regions and contain about 317 species that are also under threats. The persistence and recovery of coral communities depend partly on the success of reproduction after the disturbances. It is, therefore, important to study their reproductive biology in order to propose the effective strategy for the management of the coral reef ecosystem. Most of the previous studies documenting the sexual reproduction in scleractinian corals, however, concerned mainly the tropical-living species. To date, among the reported species in Taiwan, only 56 species including a model species, Euphyllia ancora, have been investigated. Little was known from other species. The objectives of this dissertation are 1) to study the coral spawning in the subtropical coral communities in northern Taiwan by comprehensive field samplings and observations conducted from 2013 and 2016, and by the assessments from histological analyses of the collected samples; 2) to characterize the sexual system, mode of reproduction as well as gametogenesis of Porites lichen living in Pitoujiiao, northern Taiwan by histological analyses and in situ aquarium observations; 3) to investigate the possible roles of the Dmrt gene family in coral reproduction by using Euphyllia ancora as a model. First, field observations and histological examinations revealed a large annual variation of spawning pattern among species and individuals. Forty-three species classified into 19 genera and six families spawned asynchronously in the nights (between 8 pm to 10 pm) July and/or August from 2013 to 2014. In addition, corals spawned asynchronously within the same colony. Second, the combined results from the histological examinations and others revealed that the gonochoric P. lichen is a polygamodioecious brooder that displays a seasonal pattern of sexual reproduction in gametogenesis and embryogenesis. In addition, I also reported two new morphological configurations during the early stage of gametogenesis with oocytes being developed within spermaries in bisexual polyps and melanin granular cells being clustered into spermaries. Third, I successfully isolated and characterized seven paralogous Dmrt genes (EaDmrtA, EaDmrtB, EaDmrtC, EaDmrtE, EaDmrtF, EaDmrtG, and EaDmrtH) from the gonochoric E. ancora. I found that EaDmrtE is predominantly expressed in oocytes. This oocyte-specific DmrtE is not closely related to neither the DM1 gene (i.e. currently known as DmrtA; its function is possibly related to sexual reproduction) of Acropora millepora nor to Dmrt1 of vertebrates based on the result of phylogenetic analysis; these two genes are involved in sexual reproduction, especially in sex determination (e.g. vertebrate Dmrt1 gene). High levels of EaDmrtE transcripts were detected in unfertilized mature eggs, and that were retained in newly formed zygotes and, then, the levels of expression decreased during embryonic development. I suggest that this newly discovered gene possibly plays an important role during oogenesis and early embryogenesis in corals as a maternal factor. These results strongly support a complex evolutionary history for the sexual reproduction-associated Dmrt genes with the possibility that such kinds of the genes have originated multiple times early in the evolution of metazoans. The identification of female-specific genes (E. ancora DmrtE) not only provides useful biomarkers to determine the sex of gonochoric corals, but also provide materials for molecular studies on the characterization of the genetic pathways underlying the sexual reproduction in corals.